12.3.2 Cut and Paste with Other Window Applications

Under the X Window System, there exists a primary selection
containing the last stretch of text selected in an X application
(usually by dragging the mouse). Typically, this text can be inserted
into other X applications by mouse-2 clicks. The primary
selection is separate from the clipboard. Its contents are more
“fragile”; they are overwritten each time you select text with the
mouse, whereas the clipboard is only overwritten by explicit “cut”
or “copy” commands.

Under X, whenever the region is active (see Mark), the text in
the region is saved in the primary selection. This applies regardless
of whether the region was made by dragging or clicking the mouse
(see Mouse Commands), or by keyboard commands (e.g., by typing
C-SPC and moving point; see Setting Mark).

If you change the variable select-active-regions to
only, Emacs saves only temporarily active regions to the
primary selection, i.e., those made with the mouse or with shift
selection (see Shift Selection). If you change
select-active-regions to nil, Emacs avoids saving active
regions to the primary selection entirely.

To insert the primary selection into an Emacs buffer, click
mouse-2 (mouse-yank-primary) where you want to insert it.
See Mouse Commands.

MS-Windows provides no primary selection, but Emacs emulates it
within a single Emacs session by storing the selected text internally.
Therefore, all the features and commands related to the primary
selection work on Windows as they do on X, for cutting and pasting
within the same session, but not across Emacs sessions or with other
applications.